Nanohybrid-stabilized emulsions and methods of use in oil field applications

ABSTRACT

Methods of using nanohybrid-stabilized emulsions in a well are provided that include the steps of: (a) forming an emulsion comprising: (i) a nanohybrid; (ii) water or an aqueous solution; and (iii) a water-immiscible liquid; (b) introducing a well fluid comprising the emulsion into a well; and (c) after the step of introducing, modifying the nanohybrid to break the emulsion in the well. The methods can be used in various applications, such as in drilling, completion, or intervention operations.

BACKGROUND

1. Technical Field

The inventions generally relate to the field of producing crude oil ornatural gas from a well. More particularly, the inventions are directedto improved well fluids and methods for use in wells.

2. Background Art

Producing Oil & Gas

In the context of production from a well, oil (in this context referringto crude oil) and gas (in this context referring to natural gas) arewell understood to refer to hydrocarbons naturally occurring in certainsubterranean formations. A hydrocarbon is a naturally occurring organiccompound comprising hydrogen and carbon, which can be as simple asmethane (CH₄) or can be a highly complex molecule or anything inbetween. Petroleum is a complex mixture of hydrocarbons. Oil wellsusually produce oil and gas along with water.

A subterranean formation containing oil or gas is sometimes referred toas a reservoir. A reservoir is in a shape that will trap hydrocarbonsand that is covered by a relatively impermeable rock, known as cap rock.The cap rock forms a barrier or seal above and around reservoir rock sothat fluids cannot migrate beyond the reservoir. Cap rock is commonlyshale, anhydrite, or salt. In addition, gas shows from shales duringdrilling have led some shales to be targeted as gas reservoirs. Areservoir may be located under land or under the seabed off shore. Oiland gas reservoirs are typically located in the range of a few hundredfeet (shallow reservoirs) to a few tens of thousands of feet (ultra-deepreservoirs) below the surface of the land or seabed.

As used herein, “subterranean formation” refers to the fundamental unitof lithostratigraphy. A subterranean formation is a body of rock that issufficiently distinctive and continuous that it can be mapped. In thecontext of formation evaluation, the term refers to the volume of rockseen by a measurement made through the wellbore, as in a log or a welltest. These measurements indicate the physical properties of this volumeof rock, such as the property of permeability. A “zone” refers to aninterval or unit of rock along a wellbore that is differentiated fromsurrounding rocks based on hydrocarbon content or other features, suchas faults or fractures.

To produce oil or gas from a reservoir, a wellbore is drilled into asubterranean formation, which may be the reservoir or adjacent to thereservoir. The “wellbore” refers to the drilled hole, including a casedor uncased portion of the well. As used herein, the “borehole” refers tothe inside wellbore wall, that is, the rock face or wall that bounds thedrilled hole. A wellbore can have portions that are vertical andhorizontal, and it can have portions that are straight, curved, orbranched. The wellhead is the surface termination of a wellbore, whichsurface may be on land or on a seabed. As used herein, “uphole” and“downhole” and similar terms are relative to the wellhead, regardless ofwhether a wellbore portion is vertical or horizontal.

As used herein, a “well” includes at least one wellbore. A “well” caninclude a near-wellbore region of a subterranean formation surrounding aportion of a wellbore that is in fluid communication with the wellbore.As used herein, “into a well” means at least through the wellhead. Itcan include into any downhole portion of the wellbore and it can includethrough the wellbore and into a near-wellbore region.

Generally, well services include a wide variety of operations that maybe performed in oil, gas, geothermal, or water wells, such as drilling,cementing, completion, and intervention operations. These well servicesare designed to facilitate or enhance the production of desirable fluidsfrom or through a subterranean formation.

As used herein, a “well fluid” broadly refers to any fluid adapted to beintroduced into a well for any well-servicing purpose. A “well fluid”can be, for example, a drilling fluid, a cementing fluid, a treatmentfluid, or a spacer fluid. If a well fluid is to be used in a relativelysmall volume, for example less than about 200 barrels, it is sometimesreferred to in the art as a slug or a pill. Accordingly, as used hereina “well fluid” can be a slug or a pill.

Drilling and Drilling Fluids

Drilling is the process of drilling the wellbore. The well is created bydrilling a hole, usually about 5 inches (13 cm) to about 36 inches (91cm) in diameter into the earth (or seabed) with a drilling rig thatrotates a drill string with a bit attached. After the hole is drilled,sections of steel pipe, known as casing, which are slightly smaller indiameter than the borehole, are placed in at least the uppermostportions of the borehole. The casing provides structural integrity tothe newly drilled wellbore, in addition to isolating potentiallydangerous high pressure zones from each other and from the surface.

While drilling an oil or gas well, a drilling fluid is circulateddownhole through a drillpipe to a drill bit at the downhole end, outthrough the drill bit into the wellbore, and then back uphole to thesurface through the annular path between the tubular drillpipe and theborehole. The purpose of the drilling fluid is to maintain hydrostaticpressure in the wellbore, to lubricate the drill string, and to carryrock cuttings out from the wellbore.

The drilling fluid is typically oil-based. Oil-based fluids tend to havebetter lubricating properties than water-based fluids, nevertheless,other factors can mitigate in favor of using a water-based drillingfluid.

In addition, the drilling fluid may be viscosified to help suspend andcarry rock cuttings out from the wellbore. Rock cuttings can range insize from fines to chunks measured in centimeters. Carrying capacity isthe ability of a circulating drilling fluid to transport rock fragmentsout of a wellbore. Carrying capacity is an essential function of adrilling fluid, synonymous with hole-cleaning capacity and cuttingslifting. Carrying capacity is determined principally by the annularvelocity, hole angle, and flow profile of the drilling fluid, but isalso affected by mud weight, cuttings size, and pipe position andmovement.

The wellbore may pass through zones that produce water instead ofhydrocarbons. Besides being highly undesirable during the productionphase, water-producing zones can cause problems in the wellbore withcertain drilling and completion activities and associated fluids. Forexample, the water production may highly dilute the drilling or othertreatment fluid in the well. If possible, however, water production isgenerally ignored during the drilling phase.

Cementing and Hydraulic Cement Compositions

Cementing is a common well operation. For example, hydraulic cementcompositions can be used in primary cementing operations duringcompletion in which a string of pipe, such as casing or liner, iscemented in a wellbore. In performing primary cementing, a hydrauliccement composition is pumped as a fluid (typically a suspension orslurry) into the annular space between the exterior surfaces of a pipestring and the borehole (that is, the wall of the wellbore). The cementcomposition is allowed time to set in the annular space, thereby formingan annular sheath of hardened, substantially impermeable cement. Thehardened cement supports and positions the pipe string in the wellboreand bonds the exterior surfaces of the pipe string to the walls of thewellbore. Hydraulic cement compositions can also be utilized in remedialcementing operations, such as in plugging highly permeable zones orfractures in near-wellbore regions, plugging cracks or holes in pipestrings, and the like.

Hydraulic cement is a material that when mixed with water hardens orsets over time because of a chemical reaction with the water. Becausethis is a chemical reaction with the water, hydraulic cement is capableof setting even under water. The hydraulic cement, water, and any othercomponents are mixed to form a hydraulic cement composition in theinitial state of a slurry, which should be a fluid for a sufficient timebefore setting for pumping the composition into the wellbore and forplacement in a desired downhole location in the well.

Well Treatments and Treatment Fluids

Completion is the process of making a well ready for production orinjection. This principally involves preparing the bottom of the hole tothe required specifications, running in the production tubing andassociated downhole tools, as well as perforating and stimulating asrequired.

Well intervention, or “well work,” is any operation carried out on awell during or at the end of its productive life that alters the stateof the well or well geometry, provides well diagnostics, or manages theproduction of the well. Workover can broadly refer to any kind of wellintervention involving invasive techniques, such as wireline, coiledtubing, or snubbing. More specifically, though, it refers to the processof pulling and replacing a completion.

Drilling, completion, and intervention operations can include varioustypes of treatments that are commonly performed on a well orsubterranean formation. For example, a treatment for fluid-loss controlcan be used during any of drilling, completion, and interventionoperations. During completion or intervention, stimulation is a type oftreatment performed to enhance or restore the productivity of oil andgas from a well. Stimulation treatments fall into two main groups:hydraulic fracturing and matrix treatments. Fracturing treatments areperformed above the fracture pressure of the subterranean formation tocreate or extend a highly permeable flow path between the formation andthe wellbore. Matrix treatments are performed below the fracturepressure of the formation. Other types of completion or interventiontreatments can include, for example, gravel packing, consolidation, andcontrolling excessive water production.

As used herein, the word “treatment” refers to any treatment forchanging a condition of a wellbore or an adjacent subterraneanformation. Examples of treatments include fluid-loss control, isolation,stimulation, or conformance control, however, the word “treatment” doesnot necessarily imply any particular treatment purpose.

A treatment usually involves introducing a treatment fluid into a well.As used herein, a “treatment fluid” is a fluid used in a treatment. Theword “treatment” in the term “treatment fluid” does not necessarilyimply any particular action by the fluid. If a treatment fluid is to beused in a relatively small volume, for example less than about 200barrels, it is sometimes referred to in the art as a slug or a pill.Accordingly, as used herein the term “treatment fluid” can be a slug ora pill.

The following are some examples and general descriptions of common welltreatments and associated treatment fluids. Of course, other welltreatments and treatment fluids are known in the art.

Well Treatment—Fluid-Loss Control

“Fluid loss” refers to the undesirable leakage of the liquid phase of awell fluid that contains solid particles into the matrix of asubterranean formation surrounding a portion of the wellbore. Theresulting buildup of solid particulate material on the walls of thewellbore is referred to as a filter cake. Depending on the particulatematerial and the liquid phase, such a filtercake may help block thefurther loss of the liquid phase (referred to as a filtrate) into thesubterranean formation. Providing effective fluid-loss control for wellfluids is usually highly desirable.

Fluid-loss control materials are additives specifically designed tolower the volume of a filtrate that passes through a filter medium. Mostattain their fluid-loss control from the presence of solvent-specificsolids, or from hydrated linear polymers that rely on filter cakebuildup and on viscoelasticity to inhibit fluid flow into and throughthe formation. A variety of fluid-loss control materials have been usedand evaluated, including foams, oil-soluble resins, acid-solubleparticulates, graded salt slurries, linear viscoelastic polymers, andheavy metal-crosslinked polymers. Their respective comparative effectsare well documented. Fluid-loss control materials are sometimes used indrilling fluids or treatment fluids.

Other techniques that have been developed to control fluid loss includethe use of “fluid-loss control pills,” which sometimes are referred toas “lost circulation pills.” A “fluid-loss control pill” is a treatmentfluid that is designed or used to provide some degree of fluid-losscontrol. Through a combination of viscosity, solids bridging, and cakebuildup on the porous rock, these pills oftentimes are able tosubstantially seal off portions of the formation from fluid loss. Theyalso generally enhance filter-cake buildup on the face of the formationto inhibit fluid flow into the formation from the wellbore.

Fluid-loss control pills typically comprise an aqueous base fluid and ahigh concentration of a gelling agent polymer (usually crosslinked), andsometimes, bridging particles, like graded sand, potassium salts, orsized calcium carbonate particles. The most commonly used fluid-losscontrol pills contain high concentrations (100 to 150 lbs/1000 gal) ofderivatized hydroxyethylcellulose (“HEC”). HEC is generally accepted asa gelling agent affording minimal permeability damage during completionoperations. Normally, HEC polymer solutions do not form rigid gels, butcontrol fluid loss by a viscosity-regulated or filtration mechanism.Some other gelling agent polymers that have been used include guar, guarderivatives, carboxymethylhydroxyethylcellulose (“CMHEC”), and evenstarch.

As an alternative to forming linear polymeric gels for fluid-losscontrol, crosslinked gels often are used. Crosslinking the gelling agentpolymer creates a gel structure that can support solids as well asprovide fluid-loss control. Further, crosslinked fluid-loss controlpills have demonstrated that they require relatively limited invasion ofthe formation face to be fully effective. To crosslink the gelling agentpolymers, a suitable crosslinking agent that comprises polyvalent metalions is used. Aluminum, titanium, and zirconium are common examples.

A preferred crosslinkable gelling agent for fluid-loss control pills aregraft copolymers of a hydroxyalkyl cellulose, guar, or hydroxypropylguar that are prepared by a redox reaction with vinyl phosphonic acid.The gel is formed by hydrating the graft copolymer in an aqueoussolution containing at least a trace amount of at least one divalentcation. The gel is crosslinked by the addition of a Lewis base orBronsted-Lowrey base so that pH of the aqueous solution is adjusted fromslightly acidic to slightly basic. Preferably, the chosen base issubstantially free of polyvalent metal ions. The resulting crosslinkedgel demonstrates shear-thinning and rehealing properties that providerelatively easy pumping, while the rehealed gel provides good fluid-losscontrol upon placement. This gel can be broken by reducing the pH of thefluid. Some fluid-loss pills of this type are described in U.S. Pat. No.5,304,620, assigned to Halliburton Energy Services, the relevantdisclosure of which is incorporated herein by reference. Fluid-losscontrol pills of this type are commercially available under the tradename “K-MAX” from Halliburton Energy Services Inc. in Duncan, Okla.

After their application, fluid-loss control pills can cause damage tothe permeability of near-wellbore areas due to polymer filtration orfilter-cake formation. To produce oil or gas from a subterraneanformation, the filter cake resulting from a fluid-loss control pill mustbe removed to restore the formation's permeability, preferably to atleast its original level. If the formation permeability is not restoredto at least its original level, production levels from the formation canbe significantly lower.

Well Treatment—Acidizing

A widely used stimulation technique is acidizing, in which a treatmentfluid including an aqueous acid solution is introduced into theformation to dissolve acid-soluble materials that clog or constrictformation channels. In this way, hydrocarbon fluids can more easily flowfrom the formation into the well. In addition, an acid treatment canfacilitate the flow of injected treatment fluids from the well into theformation.

Acidizing techniques can be carried out as “matrix acidizing” proceduresor as “acid fracturing” procedures.

In matrix acidizing, the acidizing fluid is injected from the well intothe formation at a rate and pressure below the pressure sufficient tocreate a fracture in the formation. The acid permeates into channels anddissolves materials that clog or constrict the channels, therebyincreasing permeability of the formation. Thus, an increase inpermeability is affected primarily by the reaction of the acid withinthe formation, and little or no permeability increase is due to inducedfractures within the formation.

In acid fracturing, an increase in permeability is affected by fracturesas well as by the acid etching through the channels within theformation. The acidic fracturing fluid is injected into the well that isdisposed within the formation to be fractured. Sufficient pressure isapplied to the acidizing treatment fluid to cause production of one ormore fractures in the formation.

Well Treatment—Hydraulic Fracturing

“Hydraulic fracturing,” sometimes simply referred to as “fracturing,” isa common stimulation treatment. A treatment fluid adapted for thispurpose is sometimes referred to as a “fracturing fluid.” The fracturingfluid is pumped at a sufficiently high flow rate and pressure into thewellbore and into the subterranean formation to create or enhance afracture in the subterranean formation. Creating a fracture means makinga new fracture in the formation. Enhancing a fracture means enlarging apre-existing fracture in the formation.

A “frac pump” is used for hydraulic fracturing. A frac pump is ahigh-pressure, high-volume pump. Typically, a frac pump is apositive-displacement reciprocating pump. The structure of such a pumpis resistant to the effects of pumping abrasive fluids, and the pump isconstructed of materials that are resistant to the effects of pumpingcorrosive fluids. Abrasive fluids include hard, insoluble particulates,such as sand, and corrosive fluids include, for example, acids. Thefracturing fluid may be pumped down into the wellbore at high rates andpressures, for example, at a flow rate in excess of 50 barrels perminute (2,100 U.S. gallons per minute) at a pressure in excess of 5,000pounds per square inch (“psi”). The pump rate and pressure of thefracturing fluid may be even higher, for example, flow rates in excessof 100 barrels per minute and pressures in excess of 10,000 psi arecommon.

To fracture a subterranean formation typically requires hundreds ofthousands of gallons of fracturing fluid. Further, it is often desirableto fracture at more than one downhole location of a well. Thus, a highvolume of fracturing fluid is usually required to treat a well, whichmeans that a low-cost fracturing fluid is desirable. Because of theready availability and relative low cost of water compared to otherliquids, a fracturing fluid is usually water-based.

The formation or extension of a fracture in hydraulic fracturingtypically occurs suddenly. When this happens, the fracturing fluidsuddenly has a fluid flow path through the fracture to flow more rapidlyaway from the wellbore. As soon as the fracture is created or enhanced,the sudden increase in the flow of fluid away from the well reduces thepressure in the well. Thus, the creation or enhancement of a fracture inthe formation is indicated by a sudden drop in fluid pressure, which canbe observed at the wellhead.

A newly-created or extended fracture will tend to close together afterthe pumping of the fracturing fluid is stopped. To prevent the fracturefrom closing, a material must be placed in the fracture to keep thefracture propped open. A material used for this purpose is referred toas a “proppant.”

The proppant is in the form of a solid particulate, which can besuspended in the fracturing fluid, carried downhole, and deposited inthe fracture as a “proppant pack.” The proppant pack props the fracturein an open condition while allowing fluid flow through the permeabilityof the pack. A particulate for use as a proppant is selected based onthe characteristics of size range, crush strength, and insolubility.

The proppant is an appropriate size to prop open the fracture and allowfluid to flow through the proppant pack, that is, in between and aroundthe proppant making up the pack. Appropriate sizes of particulate foruse as a proppant are typically in the range from about 8 to about 100U.S. Standard Mesh. A typical proppant is sand sized, which geologicallyis defined as having a largest dimension ranging from 0.0625 millimetersup to 2 millimeters (mm). (The next smaller particle size class belowsand sized is silt, which is defined as having a largest dimensionranging from less than 0.0625 mm down to 0.004 mm.) Preferably, theproppant has a particulate size distribution range such that at least90% of the proppant has a size of 0.0625 mm to 1.0 mm. For this purpose,“proppant” does not mean or refer to suspended solids, silt, fines, orother types of insoluble particulate smaller than 0.0625 mm. Further, itdoes not mean or refer to particulates larger than 2 mm.

The proppant is sufficiently strong, that is, has a sufficientcompressive or crush resistance, to prop the fracture open without beingdeformed or crushed by the closure stress of the fracture in thesubterranean formation. For a proppant material that crushes underclosure stress, the proppant preferably has an API crush strength of atleast 4,000 psi closure stress based on 10% crush fines for 20/40 meshproppant or 16% crush fines for 12/20 mesh proppant according toprocedure API RP-56. This performance is that of a medium crush-strengthproppant, whereas a very high crush-strength proppant would be 10,000psi. The higher the closing pressure of the formation of the fracturingapplication, the higher the strength of proppant is needed.

Further, a suitable proppant should not dissolve in fluids commonlyencountered in a well environment. Preferably, a material is selectedthat will not dissolve in water or crude oil.

Suitable proppant materials include, but are not limited to, sand(silica), ground nut shells or fruit pits, sintered bauxite, glass,plastics, ceramic materials, processed wood, resin coated sand or groundnut shells or fruit pits or other composites, and any combination of theforegoing. Mixtures of different kinds or sizes proppants can be used aswell. If sand is used, it typically will be from about 20 to about 100U.S. Standard Mesh in size. For a synthetic proppant, mesh sizes fromabout 8-100 typically are used.

The proppant pack in the fracture provides a higher-permeability flowpath for the oil or gas to reach the wellbore compared to thepermeability of the surrounding subterranean formation. This flow pathincreases oil and gas production from the subterranean formation.

The concentration of proppant in the treatment fluid is preferably inthe range of from about 0.03 kilograms to about 12 kilograms of proppantper liter of liquid phase (from about 0.1 lb/gal to about 25 lb/gal).

Well Treatment—Gravel Packing

An insoluble solid particulate also can be used for “gravel packing”operations. The insoluble particulate, when used for this purpose, isreferred to as “gravel.” More particularly in the oil and gas field andas used herein, the term “gravel” is sometimes used to refer torelatively-large insoluble particles in the sand size classification,that is, particles ranging in diameter from about 0.5 mm up to about 2mm. Generally, low-strength proppants are used in gravel packingincluding sand.

Increasing Viscosity of Fluid for Suspending Particulate

Various particulates can be employed in a fluid for use in a well or afluid can be used to help remove particulates from a well. As usedherein, “particulate” or “particulate material” refers to matter in thephysical form of distinct particles. The distinct particles have a hightendency to disperse. This tendency may be, for example, because theparticles have already been dispersed (e.g., the water molecules ofsteam) or because the distinct particles are too large to be appreciablyaffected by Intermolecular Forces (e.g., dust or sand). In the contextof oil and gas wells, a particulate can be a material that has particlesizes ranging anywhere from or between that of fines (measured inmicrometers) and that of rock cuttings (up to a few centimeters).

For example, during drilling, rock cuttings should be carried by thedrilling fluid and flowed out of the wellbore. The rock cuttingstypically have specific gravity greater than 2.

Similarly, a proppant used in hydraulic fracturing typically has a muchdifferent density than water. For example, sand has a specific gravityof about 2.7, where water has a specific gravity of 1.0 at roomtemperature and pressure. A proppant having a different density thanwater will tend to separate from water very rapidly.

Increasing the viscosity of the water using a viscosity-increasing agentcan help prevent a particulate having a different specific gravity thanan external phase of the fluid from quickly separating out of theexternal phase.

Emulsion for Increasing Viscosity

The internal-phase droplets of an emulsion disrupt streamlines andrequire more effort to get the same flow rate. Thus, an emulsion tendsto have a higher viscosity than the external phase of the emulsion wouldotherwise have by itself. This property of an emulsion can be used tohelp suspend a particulate material in an emulsion. This technique forincreasing the viscosity of a liquid can be used separately or incombination with other techniques for increasing the viscosity of afluid.

Viscosity-Increasing Agent

A viscosity-increasing agent is sometimes known in the art as a“thickener” or a “suspending agent,” but it should be understood thatincreasing the viscosity, without more, may only slow the settling orseparation of distinct phases.

Some viscosity-increasing agents can also help suspend a particulatematerial by increasing the elastic modulus of the fluid. An example of aviscosity-increasing agent that also increases the elastic modulus of afluid is a viscoelastic surfactant. An elastic modulus is the measure ofa substance's tendency to be deformed non-permanently when a force isapplied to it. The elastic modulus of a fluid, commonly referred to asG′, is a mathematical expression and defined as the slope of a stressversus strain curve in the elastic deformation region. G′ is expressedin units of pressure, for example, Pa (Pascals) or dynes/cm². As a pointof reference, the elastic modulus of water is negligible and consideredto be zero.

Water-Soluble Polysaccharides or Derivatives for Increasing Viscosity

A water-soluble polysaccharide can be used to increase the viscosity ofa fluid. In general, the purpose of using such a polysaccharide is toincrease the ability of the fluid to suspend and carry a particulatematerial.

A polysaccharide can be classified as being non-helical or helical (orrandom coil type) based on its solution structure in aqueous liquidmedia. Examples of non-helical polysaccharides include guar, guarderivatives, and cellulose derivatives. Examples of helicalpolysaccharides include xanthan, diutan, and scleroglucan, andderivatives of any of these.

As used herein, a “polysaccharide” can broadly include a modified orderivative polysaccharide. As used herein, “modified” or “derivative”means a compound or substance formed by a chemical process from a parentcompound or substance, wherein the chemical skeleton of the parentexists in the derivative. The chemical process preferably includes atmost a few chemical reaction steps, and more preferably only one or twochemical reaction steps. As used herein, a “chemical reaction step” is achemical reaction between two chemical reactant species to produce atleast one chemically different species from the reactants (regardless ofthe number of transient chemical species that may be formed during thereaction). An example of a chemical step is a substitution reaction.Substitution on a polymeric material may be partial or complete.

A guar derivative can be selected from the group consisting of, forexample, a carboxyalkyl derivative of guar, a hydroxyalkyl derivative ofguar, and any combination thereof. Preferably, the guar derivative isselected from the group consisting of carboxymethylguar,carboxymethylhydroxyethylguar, hydroxyethylguar,carboxymethylhydroxypropylguar, ethylcarboxymethylguar, andhydroxypropylmethylguar.

A cellulose derivative can be selected from the group consisting of, forexample, a carboxyalkyl derivative of cellulose, a hydroxyalkylderivative of cellulose, and any combination thereof. Preferably, thecellulose derivative is selected from the group consisting ofcarboxymethylcellulose, carboxymethylhydroxyethylcellulose,hydroxyethylcellulose, methylcellulose, ethylcellulose,ethylcarboxymethylcellulose, and hydroxypropylmethylcellulose.

As used herein, a polysaccharide is considered to be water soluble if itis soluble to the extent of at least 10 mg per liter in deionized waterat 25° C. More preferably, the water-soluble polymer is also soluble tothe extent of at least 10 mg per liter in an aqueous sodium chloridesolution of 32 grams sodium chloride per liter of deionized water at 25°C. If desired, the water-soluble polymer can be mixed with a surfactantto facilitate its solubility in the water or salt solution utilized. Thewater-soluble polymer can have an average molecular weight in the rangeof from about 50,000 to 20,000,000, most preferably from about 100,000to about 3,000,000.

Crosslinking of Polysaccharide to Increase Viscosity of a Fluid or Forma Gel

Because of the high volume of fracturing fluid typically used in afracturing operation, it is desirable to efficiently increase theviscosity of fracturing fluids to the desired viscosity using as littleviscosity-increasing agent as possible. Being able to use only a smallconcentration of the viscosity-increasing agent requires a lesser amountof the viscosity-increasing agent in order to achieve the desired fluidviscosity in a large volume of fracturing fluid. Efficient andinexpensive viscosity-increasing agents include water-soluble polymers.Typical water-soluble polymers used in well treatments are water-solublepolysaccharides and water-soluble synthetic polymers (e.g.,polyacrylamide, etc.). The most common water-soluble polysaccharideemployed in well treatments is guar and its derivatives.

The viscosity of a fluid at a given concentration ofviscosity-increasing agent can be greatly increased by crosslinking theviscosity-increasing agent. A crosslinking agent, sometimes referred toas a crosslinker, can be used for this purpose. One example of acrosslinking agent is the borate ion. If a polysaccharide is crosslinkedto a sufficient extent, it can form a gel with water. Gel formation isbased on a number of factors including the particular polymer andconcentration thereof, the particular crosslinker and concentrationthereof, the degree of crosslinking, temperature, and a variety of otherfactors known to those of ordinary skill in the art.

A “base gel” is a fluid that includes a viscosity-increasing agent, suchas guar, but that excludes crosslinking agents. Typically, a base gel isa fluid that is mixed with another fluid containing a crosslinker,wherein the mixed fluid is adapted to form a gel after injectiondownhole at a desired time in a well treatment. A base gel can be used,for example, as the external phase of an emulsion.

Breaker for Polysaccharide or Crosslinked Polysaccharide

Drilling or treatment fluids also commonly include a “breaker” for apolysaccharide or crosslinked polysaccharide. In this context ofviscosity increase provided by a use of a polysaccharide, the term“break” or “breaker” as used herein refers to a reduction in theviscosity of a fluid or gel by some breaking of the polymer backbones orsome breaking or reversing of the crosslinks between polymer molecules.No particular mechanism is necessarily implied by the term. A breakerfor this purpose can be, for example, an acid, base, an oxidizer, anenzyme, chelating agent of metal crosslinker or a combination of these.The acids, oxidizers, or enzymes can be in the form of delayed-releaseor encapsulated breakers.

In the case of a crosslinked viscosity-increasing agent, for example,one way to diminish the viscosity is by breaking the crosslinks. Forexample, the borate crosslinks in a borate-crosslinked gel can be brokenby lowering the pH of the fluid. At a pH above 8, the borate ion existsand is available to crosslink and cause gelling. At a lower pH, theborate ion reacts with proton and is not available for crosslinking,thus, an increase in viscosity due to borate crosslinking is reversible.

Other Uses of Polysaccharide in Well Fluids

There are other uses for a water-soluble polysaccharide in a wellfluids. For example, during the drilling, completion, and stimulation ofsubterranean a well, it is common to pump a water-based well fluidthrough tubular goods (e.g., pipes, coiled tubing, etc.) and into asubterranean formation adjacent a wellbore. A considerable amount ofenergy may be lost due to friction of the water-based well fluid inturbulent flow through the tubular goods of the wellbore. Because ofthese energy losses, additional pumping horsepower may be necessary toachieve the desired purpose of the well fluid. To reduce these energylosses, a water-soluble polysaccharide may be included in a water-basedwell fluid. The use of an appropriate water-soluble polysaccharide as afriction reducer in a well fluid is expected to reduce the energy lossesdue to friction.

For example, in a “high-rate water fracturing treatment,” proppantsuspension in the treatment fluid is largely achieved by the high rateof pumping and the high flow rate of the treatment fluid. To reduceenergy losses due to friction, a water-soluble polysaccharide as afriction reducer may be included in the fracturing fluid. While a fluidused in high-rate water fracturing may contain a water-solublepolysaccharide as a friction-reducing polymer, the polysaccharide isusually included in the fracturing fluid in an amount that is sufficientto provide the desired friction reduction without appreciablyviscosifying the fluid and usually without a crosslinker. As a result,the fracturing fluids used in these high-rate water-fracturingoperations generally have a lower viscosity than conventional fracturingfluids.

Spacer Fluids

A spacer fluid is a fluid used to physically separate onespecial-purpose fluid from another. Special-purpose fluids are typicallyprone to contamination, so a spacer fluid compatible with each is usedbetween the two. A spacer fluid is used when changing well fluids usedin a well. For example, a spacer fluid is used to change from a drillingfluid during drilling a well to a cement slurry during cementingoperations in the well. In case of an oil-based drilling fluid, itshould be kept separate from a water-based cementing fluid. In changingto the latter operation, a chemically treated water-based spacer fluidis usually used to separate the drilling fluid from the cement slurry.Another example is using a spacer fluid to separate two differenttreatment fluids.

Well Fluid Additives

A well fluid can contain additives that are commonly used in oil fieldapplications, as known to those skilled in the art. These include, butare not necessarily limited to, inorganic water-soluble salts, breakeraids, surfactants, oxygen scavengers, alcohols, scale inhibitors,corrosion inhibitors, fluid-loss additives, oxidizers, and bactericides.

Variations in Well Fluid Over Time

Unless the specific context otherwise requires, a “well fluid” refers tothe specific properties and composition of a fluid at the time the fluidis being introduced through the wellhead into a wellbore. In addition,it should be understood that, during the course of a well operation suchas drilling, cementing, completion, or intervention, or during aspecific treatment such as fluid-loss control, hydraulic fracturing, ora matrix treatment, the specific properties and composition of a type ofwell fluid can be varied or several different types of well fluids canbe used. For example, the compositions can be varied to adjust viscosityor elasticity of the well fluids to accommodate changes in theconcentrations of proppant desired to be carried down to thesubterranean formation from initial packing of the fracture to tail-endpacking. It can also be desirable to accommodate expected changes intemperatures encountered by the well fluids during the course of thetreatment. By way of another example, it can be desirable to accommodatethe longer duration that the first treatment fluid may need to maintainviscosity before breaking compared to the shorter duration that alater-introduced treatment fluid may need to maintain viscosity beforebreaking. Changes in concentration of the proppant, viscosity-increasingagent, or other additives in the various treatment fluids of a treatmentoperation can be made in stepped changes of concentrations or rampedchanges of concentrations.

Continuum Mechanics and Rheology

One of the purposes of identifying the physical state of a substance andmeasuring the viscosity of a fluid substance is to establish whether itis pumpable under the ranges of physical conditions that may beencountered at a wellhead and with the types and sizes of pumpsavailable to be used for pumping fluids into a well. Another purpose isto determine what the physical state of the substance and its physicalproperties will be during pumping through a wellbore and under otherdownhole conditions in the well, including over time and changingtemperatures, pressures, and shear rates. For example, in someapplications, a well fluid forms or becomes a gel under downholeconditions that later is broken back to a fluid state.

Continuum mechanics is a branch of mechanics that deals with theanalysis of the kinematics and the mechanical behavior of materialsmodeled as a continuous mass rather than as discrete particles. Rheologyis the study of the flow of matter: primarily in the liquid state, butalso as “soft solids” or solids under conditions in which they respondwith plastic flow rather than deforming elastically in response to anapplied force. It applies to substances that have a complex structure,such as muds, sludges, suspensions, gels, etc. The flow of suchsubstances cannot be characterized by a single value of viscosity, whichvaries with temperature, pressure, and other factors. For example,ketchup can have its viscosity reduced by shaking (or other forms ofmechanical agitation) but water cannot.

As used herein, if not other otherwise specifically stated, the physicalstate of a substance (or mixture of substances) and other physicalproperties are determined at a temperature of 77° F. (25° C.) and apressure of 1 atmosphere (Standard Laboratory Conditions).

Fluid State and Viscosity

In general, a fluid is an amorphous substance that is or has acontinuous phase and that tends to flow and to conform to the outline ofits container. Examples of fluids are gases and liquids. A gas (in thesense of a physical state) refers to an amorphous substance that has ahigh tendency to disperse and a relatively high compressibility. Aliquid refers to an amorphous substance that has little tendency todisperse and relatively high incompressibility. The tendency to disperseis related to Intermolecular Forces (also known as van der Waal'sForces).

Viscosity is the resistance of a fluid to flow. In everyday terms,viscosity is “thickness” or “internal friction.” Thus, pure water is“thin,” having a relatively low viscosity whereas honey is “thick,”having a relatively higher viscosity. Put simply, the less viscous thefluid is, the greater its ease of movement (fluidity). More precisely,viscosity is defined as the ratio of shear stress to shear rate. Theviscosity of a fluid is usually expressed in units of centipoise (“cP”).

The physical state of a substance and the viscosity of a fluid arehighly dependent on the nature of the substance and the physicalconditions, primarily temperature and pressure. In addition, thephysical state and the viscosity of a fluid may depend on shear stressand shear rate and the viscosity may vary over time with continuingshear. Newton's law of viscosity is an approximation that holds for somesubstances but not others. Non-Newtonian fluids exhibit a morecomplicated relationship between shear stress and velocity gradient thansimple linearity. Thus, there exist a number of forms of viscosity.Newtonian fluids, such as water and most gases, have a constantviscosity with rate of shear. Shear thickening fluids have a viscositythat increases with the rate of shear. Shear thinning fluids have aviscosity that decreases with the rate of shear. Thixotropic fluidsbecome less viscous over time when shaken, agitated, or otherwisestressed. Rheopectic fluids become more viscous over time when shaken,agitated, or otherwise stressed. A Bingham plastic is a material thatbehaves as a solid at low stresses but flows as a viscous fluid at highstresses.

There are numerous ways of measuring and modeling viscous properties,and new developments continue to be made. The methods depend on the typeof fluid for which viscosity is being measured. A typical method forquality assurance or quality control (QA/QC) purposes uses a couettedevice, such as a Fann Model 50 viscometer, that measures viscosity as afunction of time, temperature, and shear rate. The viscosity-measuringinstrument can be calibrated using standard viscosity silicone oils orother standard viscosity fluids. Due to the geometry of most commonviscosity-measuring devices, however, large particles of solidparticulate, such as proppant or gravel used in certain well treatments,would interfere with the measurement. Therefore, the viscosity of afluid containing such large solid particulate is usually inferred bymeasuring the viscosity of a test fluid that is similar to thefracturing fluid without any proppant included. Another method uses aProppant Transport Measuring Device (PTMD) (see SPE 115298), whichallows the measurement of the viscosity of a fluid containing aproppant. The PTMD instrument is preferably calibrated against a moreconventional instrument, for example, against a Fann Model 50viscometer.

Gel State and Deformation

A gel state is a semi-solid, jelly-like state that can have propertiesranging from soft and weak to hard and tough. Shearing stresses below acertain finite value fail to produce permanent deformation. The minimumshear stress which will produce permanent deformation is known as theshear or gel strength of the gel.

Substance of a Fluid or Gel can be a Dispersion

A dispersion is a system in which particles are dispersed in a externalphase of a different composition or physical state. A dispersion can beclassified a number of different ways, including based on the size ofthe dispersed-phase particles, whether or not precipitation occurs, andthe presence of Brownian motion. For example, a dispersion can beconsidered to be homogeneous or heterogeneous based on being a solutionor not, and if not a solution, based on the size of the dispersed-phaseparticles (which can also refer to droplet size in the case of adispersed liquid phase).

The substance of a fluid can be a single chemical substance or adispersion. For example, water (a liquid under Standard LaboratoryConditions) is a single chemical by that name. An aqueous salt solutionis a dispersion.

The substance of a gel is a dispersion. The gel state is formed by anetwork of interconnected molecules, such as of a crosslinked polymer orof micelles, with other molecules in liquid form. The network gives agel material its structure (hardness) and contributes to stickiness(tack). By weight, the substance of gels is mostly liquid, yet theybehave like solids due to the three-dimensional network with the liquid.At the molecular level (nanometer scale), a gel is a dispersion in whichthe network of molecules is the continuous (external) phase and theliquid is the discontinuous (internal) phase; however, the gel state,although heterogeneous, is generally considered to be a single phase.

In addition, a dispersion can be a foam, which is a liquid that includesa dispersion of undissolved gas bubbles that foam the fluid, usuallywith the aid of a chemical (a foaming agent) to achieve stability.

Classification of Dispersions: Homogeneous and Heterogeneous

A dispersion is considered to be homogeneous if the dispersed phaseparticles are dissolved in solution or the particles are less than about1 nanometer in size.

A solution is a special type of homogeneous mixture. A solution ishomogeneous because the ratio of solute to solvent is the samethroughout the solution and because solute will never settle out ofsolution, even under powerful centrifugation. An aqueous solution, forexample, saltwater, is a homogenous solution in which water is thesolvent and salt is the solute.

Except for the special case of a solution, a dispersion is considered tobe heterogeneous if the dispersed-phase particles are greater than about1 nanometer in size. (For reference, the diameter of a molecule oftoluene is about 1 nm).

Classification of Heterogeneous Dispersions: Colloids and Suspensions

Dispersions can be further classified based on particle size and othercharacteristics.

A heterogeneous dispersion is a “colloid” where the dispersed-phaseparticles are in the range of about 1 nanometer to about 1 micrometer insize. Typically, the dispersed-phase particles of a colloid have adiameter of between about 5 to about 200 nanometers. Such particles arenormally invisible to an optical microscope, though their presence canbe confirmed with the use of an ultramicroscope or an electronmicroscope.

A heterogeneous dispersion is a “suspension” where the dispersed-phaseparticles are larger than about 1 micrometer. Such particles can be seenwith a microscope, or if larger than about 0.1 mm, with the naked eye.

Classification of Colloids or Suspensions: External Phase

Colloids or suspensions can have solid, liquid, or gas as the externalphase.

In the cases where the external phase of a dispersion is a liquid, for acolloidal fluid the dispersed-phase particles are so small that they donot settle. Unlike colloids, however, a suspension of particlesdispersed in a liquid external phase will eventually separate onstanding, e.g., settle in cases where the particles have a higherdensity than the liquid phase. Suspensions having a liquid externalphase are essentially unstable from a thermodynamic point of view;however, they can be kinetically stable over a large period of time,depending on temperature and other conditions.

An example of a suspension of a solid in a liquid would be sand inwater. In case the dispersed-phase particles are liquid in an externalmedium that is another liquid, this kind of suspension is moreparticularly referred to as an emulsion. Suspensions and emulsions arecommonly used as well fluids.

Emulsions

More particularly, an emulsion is a dispersion of immiscible liquid asdroplets into an external liquid phase. In addition, the proportion ofthe external and internal phases is above the solubility of either inthe other. A chemical (an emulsifier or emulsifying agent) can beincluded to reduce the interfacial tension between the two immiscibleliquids to help with stability against coalescing of the internal liquidphase.

An emulsion can be an oil-in-water (o/w) type or water-in-oil (w/o)type. A water-in-oil emulsion is sometimes referred to as an invertemulsion. In the context of an emulsion, the “water” phase refers towater or an aqueous solution and the “oil” phase refers to any non-polarorganic liquid, such as petroleum, kerosene, or synthetic oil.

It should be understood that multiple emulsions are possible, which aresometimes referred to as nested emulsions. Multiple emulsions arecomplex polydispersed systems where both oil-in-water and water-in-oilemulsions exist simultaneously in the fluid, wherein the oil-in-wateremulsion is stabilized by a lipophillic surfactant and the water-in-oilemulsion is stabilized by a hydrophilic surfactant. These includewater-in-oil-in-water (w/o/w) and oil-in-water-in-oil (o/w/o) typemultiple emulsions. Even more complex polydispersed systems arepossible. Multiple emulsions can be formed, for example, by dispersing awater-in-oil emulsion in water or an aqueous solution, or by dispersingan oil-in-water emulsion in oil.

Classification of Fluids: Water-Based or Oil-Based

The continuous phase of a substance as a whole is the most externalphase, regardless of the number of phases. As used herein, a“water-based fluid” means that water or an aqueous solution is thecontinuous phase of the fluid as a whole. In contrast, an “oil-basedfluid” means that oil is the continuous phase of the fluid as a whole.

SUMMARY OF THE INVENTION

According to the invention, methods of using nanohybrid-stabilizedemulsions in a well are provided. The methods include the steps of:

(a) forming an emulsion comprising:

-   -   (i) a nanohybrid;    -   (ii) water or an aqueous solution; and    -   (iii) a water-immiscible liquid;

(b) introducing a well fluid comprising the emulsion into a well; and

(c) after the step of introducing, modifying of the nanohybrid to breakthe emulsion in the well.

According to the invention, it is recognized that the nanohybrid can bemodified or completely destroyed, which can be used as a “switch” toselectively break the emulsion. This can be useful where the surfaceactivity of the nanohybrid is needed for a certain period and then thatsurface activity is needed to be “turned off.”

As will be appreciated by a person of skill in the art, the methodsaccording to the invention can have application in various kindsoperations involved in the production of oil and gas, includingdrilling, completion, and intervention.

The features and advantages of the present invention will be apparent tothose skilled in the art. While numerous changes may be made by thoseskilled in the art, such changes are within the spirit of the invention.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS AND BEST MODEGeneral Definitions and Usages

As used herein, the words “comprise,” “have,” “include,” and allgrammatical variations thereof are each intended to have an open,non-limiting meaning that does not exclude additional elements or steps.

As used herein, a material is considered to be “soluble” in a liquid ifat least 10 mg of the material can be dissolved in one liter of theliquid when tested at 77° F. and 1 atmosphere pressure and considered tobe “insoluble” if less than that.

Unless otherwise specified, any ratio or percentage means by weight.

As used herein, the phrase “by weight of the water” means the weight ofthe water of the continuous phase of the fluid as a whole without theweight of any proppant, viscosity-increasing agent, dissolved salt, orother materials or additives that may be present in the water.

Unless otherwise specified, any doubt regarding whether units are inU.S. or Imperial units, where there is any difference U.S. units areintended herein. For example, “gal/Mgal” means U.S. gallons per thousandU.S. gallons.

As used herein, “first,” “second,” or “third” may be arbitrarilyassigned and are merely intended to differentiate between two or morefluids, aqueous solutions, etc., as the case may be. Furthermore, it isto be understood that the mere use of the term “first” does not requirethat there be any “second,” and the mere use of the word “second” doesnot require that there by any “third,” etc.

Unless otherwise specified, as used herein, the viscosity of a fluid ismeasured at 40 sec-1 and at room temperature of about 77° F. (25° C.).

Nanohybrids for Stabilizing Emulsions

The purpose of this invention is to use nanohybrids comprising a carbonnanotube and inorganic second component (e.g., silica, alumina,magnesium oxide, titanium oxide, etc.) for use in emulsions foroil-field applications. More particularly, these nanohybrids contain thehydrophobic carbon nanotube and the hydrophilic inorganic component(e.g., silica) attached to each other.

The inherent hydrophobic and hydrophilic character gives thesenanohybrids surface-active properties. The hydrophilic-lipophilicbalance (“HLB”) of the nanohybrid can be tailored by chemicalfunctionalization of the nanohybrid to get the desired properties. Morespecifically, the HLB can be tailored by functionalization of nanotubesto make them more hydrophilic. The nanotubes can be made progressivelymore hydrophilic to make the HLB higher assuming that in the beginningthe nanohybrid has a low HLB, i.e., it is more oil like. If made to betoo hydrophilic, however, the material will lose its surface-activeproperties as there would not be both a hydrophobic and hydrophilic partin the hybrid. This invention takes advantage of this in the forming andthen breaking of an emulsion. Similarly, if nanohybrid is made to be toohydrophobic, the material will lose its surface-active properties asthere would not be both a hydrophobic and hydrophilic part in thehybrid. This can also be used as a method to break the emulsion.

The surface-active nanohybrids partition at the interface of an aqueousphase and an oil phase. This is different from conventional surfactantsthat form micelles. The thermodynamic energy required to displaceparticles stabilized or nanohybrid stabalized emulsion from theinterface is very high in comparison to conventional surfactants.Therefore, these emulsions formed with surface-active nanohybrids aremuch more stable than conventional surfactants that form micelles.

Nanohybrids are a new class of hybrid materials is made from carbonnanotubes (CNTs) and inorganic glasses or ceramics, which are sometimesknown as CNT-inorganic hybrids. The many advantages of CNTs in hybridmaterials include their high aspect ratio (>1,000) and tubular geometry,which provides ready gas access to a large specific surface area andpercolation at very low volume fractions. CNTs have been combined with avariety of inorganic compounds, including oxides, nitrides, carbides,chalcogenides, and ceramics. In contrast to nanocomposites, the CNTs arecoaxially coated with the inorganic compound. Dominik Eder, CarbonNanotube-Inorganic Hybrids, Chem. Rev. 2010, 110, 1348-1385.

In simple terms, CNTs are tubular structures made of rolled-up layers ofinterconnected carbon atoms with diameters ranging from about onenanometer to tens of nanometers and lengths up to tens of microns. CNTscan be open-ended or closed by a hemispherical fullerene-type cap,depending on the synthesis method. Along with structures related tothose of fullerenes, CNTs are considered a third allotropic form ofcarbon, with the others being diamond and graphite. They are classifiedas either (a) “single-walled” tubes (SWCNTs, 0.7<d<2 nm), which consistof a single layer of graphene sheet seamlessly rolled into a cylindricaltube, or (b) multiwalled CNTs (MWCNT, 1.4<d<150 nm), which comprisemultiple concentric tubes separated by about 0.34 nm. In general, CNTspossess large specific surface areas due to their hollow geometry, whiletheir structural integrity and chemical inertness support relativelyhigh oxidation stability. Other advantages include their exceptionalphysical properties. Dominik Eder, Carbon Nanotube-Inorganic Hybrids,Chem. Rev. 2010, 110, 1348-1385.

In general, CNTs can be functionalized by (a) covalent attachment ofchemical groups through bonding to the π-conjugated skeleton of the CNTor (b) noncovalent adsorption or wrapping of various functionalmolecules. The CNT reactivity is directly related to the π-orbitalmismatch caused by an increased curvature. Therefore, a distinction mustbe made between the sidewall and the endcaps of a nanotube. Thesidewalls can be considered as curved graphite, while the tips are likethe structure of a fullerene hemisphere and are thus relativelyreactive. Hence, most reactions will occur first at the tips and then onthe sidewalls. Dominik Eder, Carbon Nanotube-Inorganic Hybrids, Chem.Rev. 2010, 110, 1348-1385.

The various synthesis strategies for CNT-inorganic hybrids can becategorized as ex situ and in situ techniques. The ex situ (buildingblock) approach first produces the inorganic component in the desireddimensions and morphology (typically spherical nanoparticles), thenmodifies and attaches this component to the surface of CNTs viacovalent, noncovalent, or electrostatic interactions. In contrast, thein situ approach carries out the synthesis of the inorganic component inthe presence of pristine or functionalized CNTs, onto which theinorganic material grows as particles, nanowires, or thin films. DominikEder, Carbon Nanotube-Inorganic Hybrids, Chem. Rev. 2010, 110,1348-1385.

Surface-active nanohybrids are a new class of surfactant material. Suchnanohybrids were developed by Professor Daniel Resasco at the Universityof Oklahoma, and are currently commercially available from SouthWestNanoTechnologies Inc. in Norman, Okla. See Min Shen and Daniel E.Resasco, Emulsions Stabilized by Carbon Nanotube-Silica Nanohybrids,Langmuir 2009, 25(18), 10843-10851, Jun. 17, 2009. These nanohybridshave been shown to make water-in-oil emulsions that are stable intemperature range of about 50° C. to about 250° C., pH range of 1 to 9,pressures of about 900 psi, and in salt solutions. In addition, thestability of the emulsion is not compromised by diluting the emulsionwith NaCl solution. A minuscule amount of an aqueous liquid in awater-in-oil emulsion comes out of the emulsion, but the bulk remains inthe emulsion. The nanohybrid made from multi-walled carbon nanotubes(“MWNT”) gives better performance than single-walled carbon nanotubes(“SWNT”) in stabilizing emulsions. In addition, MWNT are currently muchless expensive than SWNT, which provides a commercial advantage overSWNT.

Step of Forming an Emulsion

The invention includes a step of forming an emulsion comprising: (i) ananohybrid; (ii) water or an aqueous solution; and (iii) awater-immiscible liquid. Without being limited by any theory, it isbelieved that the nanohybrid functions to help emulsify and maintain thestability of the emulsion. As used herein, an “emulsifier” means that ithelps prevent the droplets of the internal dispersed phase fromflocculating or coalescing in the external phase. The nanohybrid helpsstabilize the emulsion, but optionally other surfactants, particulatematerials, or polymers can also be included to further enhance thestability of the emulsion.

According to an embodiment, the nanohybrid has a hydrophilic-lipophilicbalance adapted to forming an oil-in-water emulsion. According toanother embodiment, the nanohybrid has a hydrophilic-lipophilic balanceadapted to forming a water-in-oil emulsion. Generally, nanohybrids arehydrophobic prior to any derivatization and prior to any derivitizationwill make water-in-oil emulsion.

According to an embodiment, a dispersed phase of the emulsion comprisesa chemical to be released from the dispersed phase upon the step ofmodifying the nanohybrid to break the emulsion. For example, thechemical to be released can include a crosslinker or a breaker forpolysaccharide in fracturing applications in which delayed crosslinkingor breakage is desired. It can also be used to release cement retarderor accelerator for cementing applications.

Preferably, the water-immiscible liquid is water insoluble.

The emulsion can also include other additives.

The emulsion can also contain water-soluble salt(s) at a high-ionicstrength for other purposes, for example, to increase the density of thecontinuous phase of the emulsion or to prevent the swelling of the clayin the formation. Preferably, the water-soluble salt is selected fromthe group consisting of: an alkali metal halide, alkaline earth halide,alkali metal formate, and any combination thereof in any combination.

The emulsion can contain a freezing-point depressant. More preferably,the freezing point depressant is for the continuous phase of theemulsion as a whole. Preferably, the freezing-point depressant isselected from the group consisting of water-soluble ionic salts,alcohols, glycols, urea, and any combination thereof in any proportion.

The emulsion can include water-soluble salt(s) at a high-ionic strength.The method can also include the step of adding the emulsion to anaqueous solution of a high-ionic strength prior to or during the step ofintroducing.

The emulsion can include a proppant. The method can include the step ofmixing the emulsion with another fluid comprising the proppant to form atreatment fluid prior to or during the step of introducing. Preferably,the proppant is in a particulate size distribution range such that atleast 90% of the proppant has a size within the range of 0.0625 mm to2.0 mm.

Stability of the Emulsion Composition

Preferably, an emulsion composition used in a method according to thepresent invention is highly stable under a wide range of downholeconditions such that it will not cream, flocculate, or coalesce in usedownhole until the emulsion is broken. For example, as used herein, itshould be stable at 77° F. for at least 10 days. In downhole conditionsat a higher temperature, it should be stable for at least the time ofthe job until it is desired to be broken.

As used herein, the term “cream” means at least some of the droplets ofa dispersed phase converge towards the surface or bottom of the emulsion(depending on the relative densities of the liquids making up thecontinuous and dispersed phases). The converged droplets maintain adiscrete droplet form. As used herein, the term “flocculate” means atleast some of the droplets of a dispersed phase combine to form smallaggregates in the emulsion. As used herein, the term “coalesce” means atleast some of the droplets of a dispersed phase combine to form largerdrops in the emulsion. It should be understood that an emulsion isvisually examined for creaming, flocculating, or coalescing.

Preferably, an emulsion composition according to the invention is stableunder one or more of certain conditions commonly encountered in thestorage and use of such an emulsion composition for use in a well. Forexample, an emulsion composition according to the invention ispreferably stable for storing, including under freeze-thaw conditions,to high-temperature well environments, to the addition of salts to givea high-ionic strength to the water phase of the emulsion, or to dilutingthe emulsion with high concentrations of water or solutions havinghigh-ionic strength. Most preferably, an emulsion according to theinvention has all of these advantages.

As used herein, stability to storing means stability to storing at 77°F. for 10 days. As used herein, stability to “freeze-thaw conditions”means to cooling from 77° F. to 0° F. and warming back to 77° F.Preferably, the dispersed phase does not cream, flocculate, or coalescewhen tested under a freeze-thaw cycle from 77° F. to 0° F. and back to77° F. More preferably, the dispersed phase does not cream, flocculate,or coalesce when cooled and stored at a temperature of 0° F. for 10 daysand then warmed to 77° F.

As used herein, “high-temperature conditions” means in the range of 230°F.-500° F. Preferably, the emulsion does not cream, flocculate, orcoalesce when tested at for the duration of the job under thetemperature conditions of the job.

As used herein, “high-dilution conditions” means dilution with 5-20times the amount of the external phase. Preferably, the dispersed phaseof the emulsion does not cream, flocculate, or coalesce when tested bydilution with at least 5 times the amount of the external phase at 77°F. For example, the stability of a nanohybrid-stabilized oil-in-wateremulsion is preferably not compromised by diluting the emulsion 15 timeswith 1 Molar NaCl solution.

Step of Storing the Emulsion Before Use

The method can optionally include the step of storing the emulsion forat least 7 days within a temperature range of 25° F.-120° F. between thestep of forming and the step of introducing. The method can optionallyinclude the step of storing the emulsion under conditions such that theemulsion undergoes at least one freeze-thaw cycle between the step offorming and the step of introducing.

Step of Introducing Emulsion into a Well

The method can optionally include a step of mixing the emulsion withanother material to form a well fluid comprising the emulsion. A wellfluid comprising the emulsion can include the emulsion as a nestedemulsion in the fluid or the well fluid can be a dilution or othermodification of the emulsion, provided that the emulsion is not brokenbefore the step of introducing the well fluid into the well.

In an embodiment, the well fluid introduced into the well can be theemulsion.

In another embodiment, the method can comprise a step of mixing theemulsion with a third fluid to form the well fluid prior to or duringthe step of introducing the well fluid into the well. In an embodiment,the well fluid introduced into the well comprises the emulsion as anested emulsion in a third fluid that is the continuous phase of thewell fluid as a whole. The third fluid can include water and awater-soluble polysaccharide. The third fluid can include at least asufficient concentration of the water-soluble polysaccharide to becapable of forming a crosslinked gel upon mixing with a crosslinker. Thethird fluid can optionally include water and one or more inorganic ionicsalts.

In an embodiment, the well fluid can include at least one additiveselected from the group consisting of: a conventional surfactant, ananti-scaling agent, a crosslinker, corrosion inhibitor, and a breakerfor polysaccharide or crosslinked polysaccharide.

In yet another embodiment, the method can include a step of adding theemulsion to an aqueous solution of a high-ionic strength salt prior toor during the step of introducing.

According to an embodiment, the well fluid has an elastic modulus ofgreater than 1 Pa as measured within the linear viscoelastic region.

According to an embodiment, the well fluid is introduced into the wellat a rate and pressure and directed to a subterranean formation at arate and pressure that is at least sufficient to create at least onefracture in the subterranean formation of the well. The well fluid canfurther include a proppant. For example, the method can include a stepof mixing the emulsion with a third fluid comprising a proppant prior toor during the step of introducing.

The step of introducing a well fluid comprising the emulsion into a wellcan be accomplished by pumping or injecting according to varioustechniques well known in the art.

Step of Breaking the Emulsion

According to the invention, it is recognized that the surface activityof these nanohybrid surfactants can be modified or completely destroyed,which property can be used as a “switch” for breaking an emulsionstabilized by a nanohybrid surfactant.

For example, oxidizing the carbon nanotube component and making it morehydrophilic can change the surface activity. An example of a suitableoxidizer is a nitric acid/sulfuric acid mixture to generate hydroxyl andcarboxyl groups on the nanotubes to make them hydrophilic. It is alsocontemplated that the silica of the nanohybrid can be chemically removedor made oil-wet making it more hydrophilic. For example, silica can bereacted with long chain quaternary amine compounds to make themhydrophobic thereby making the whole of the nanohybrid hydrophobic. Thiswill result in dissolution of the nanohybrid in the oil phase, thusbreaking the emulsion. Either approach can be used as a “switch” usefulin a variety of oil field applications, where surface activity is neededfor a certain period and then that surface activity is needed to beturned off. The silica can also be reacted with silanes to make themhydrophobic or reacted with epoxides containing a long chain alkyl groupto make them hydrophobic.

The emulsion can also be broken by addition of chemicals thatfunctionalize the nanotube or form charge-transfer complexes. Ahydrophilic group can be attached with the moiety that forms acharge-transfer complex with the carbon nanotube.

An emulsion stabilized with a nanohybrid can also be broken by wrappingthe nanohybrid in a water-soluble polymer.

For a nanohybrid made up of some inorganic material other than silica, asuitable chemical group can be attached to make it more hydrophobic tobreak emulsion.

In some cases, these materials for modifying or destroying thenanohybrid can be included at the time of preparation of an emulsion. Inother cases, the material can be pumped later to break the emulsion.

After the step of introducing, the methods include a step of modifyingof the nanohybrid to break the emulsion in the well.

As used herein, to “break” an emulsion means to cause the creaming andcoalescence of emulsified drops of the internal dispersed phase so thatthey the internal phase separates out of the external phase. Breaking anemulsion can be accomplished mechanically (for example, in settlers,cyclones, or centrifuges) or with chemical additives to increase thesurface tension of the internal droplets.

Preferably, the step of modifying the nanohybrid is by modifying thehydrophilic-lipophilic balance (“HLB”) of the nanohybrid such that itwould no longer stabilize the original emulsion.

Preferably, according to one embodiment, the step of modifying thenanohybrid is with a strong oxidizing agent for the nanohybrid. Theemulsion can be broken by treating the emulsion with an oxidizer such asnitric acid. The oxidizer functionalizes the carbon nanotubes of thenanohybrid and reduces their hydrophobic surface activity, therebybreaking the emulsion. Alternatively, an oxidizer can be incorporated inthe aqueous phase of the emulsion and the emulsion can be tailored toself-degrade after a delay. An example of a suitable oxidizing agent isHNO₃.

According to an embodiment, the well fluid comprises a chemical formodifying the hydrophilic-lipophilic balance of the nanohybrid.According to another embodiment, a chemical for modifying thehydrophilic-lipophilic balance of the nanohybrid is pumped separatelyfrom the well fluid, for example, as a post flush over the well fluid ora pre-flush that flows back over the well fluid.

Applications

These nanohybrid emulsions can be used for various oil fieldapplications described below, such as in drilling, completion, orintervention operations.

Drilling Fluid and Drilling

The nanohybrids can be used to make emulsion-based well fluids fordrilling and other application that can be used in the temperature rangeof 22° C. to 250° C. (71.6° F. to 482° F.). For drilling operations,water-in-oil emulsions are typically desired; however, oil-in-wateremulsions are sometimes used. Oil-in-water emulsions are used in certainformations where oil wetting of the formation face in not desired. Theemulsion can also be used below 22° C. where winterization of theemulsion is undertaken by addition of salt to water phase or addingglycols or alcohols to the aqueous phase of the emulsion. For example,the stability of an oil-in-water emulsion is not compromised by dilutingthe emulsion (e.g., 15 times with 1 Molar NaCl solution). This is anadvantage while drilling through a water-bearing formation, whereintegrity of the emulsion would otherwise be compromised by dilutionwith the invading water.

The emulsion can be broken by treating the emulsion with a chemical tochange the surface activity of the nanohybrid according to one of thetechniques described above, thereby breaking the emulsion. This is a wayto remove a filter cake built by these drilling fluids. Alternatively,an chemical can be incorporated in the aqueous phase of the emulsion andthe emulsion can be tailored to self-degrade after a delay.

Cementing Fluid and Cementing

An emulsion stabilized with a nanohybrid can be used to deliver anaccelerator for delayed setting of a cement in a cementing application.The emulsion can be part of a cementing fluid. For example, the cementaccelerator such as calcium chloride can be solubilized in the aqueousphase and then emulsion is prepared with nanohybrid and oil. The aqueousphase can optionally have breaker to break the emulsion to releasecalcium chloride accelerator. Similarly oxidizer can be encapsulated inthe water-in-oil emulsion which can be broken down in cement bytechniques described previously thereby releasing oxidizer that reactwith cement retarders present in the cement slurry to accelerate thesetting of the cement. The emulsion can be broken by treating with achemical to change the surface activity of the nanohybrid according toone of the techniques described above, thereby breaking the emulsionwhen desired to release the retarder or accelerator.

Fluid-Loss Control Pill and Fluid-Loss Control

An emulsion stabilized with a nanohybrid can be used in a fluid-losscontrol pill. In the fluid-loss pill the emulsion can contain oxidizerfor breakage of the fluid-loss pill polymer and slowly release thebreaker to break the pill. The emulsion can be broken by treating with achemical to change the surface activity of the nanohybrid according toone of the techniques described above, thereby breaking the emulsionwhen desired. The emulsion can also be used to incorporate a crosslinkerin the emulsion that may release slowly to crosslink the polymer. Thiswill give the time to place the pill at a desired downhole location inthe wellbore. These pills are very thick (e.g., 110 lb/Mgal crosslinkedgels) and difficult to pump if fully crosslinked at the surface.Delaying the crosslinking until the pill reaches the downhole locationis desirable. In other embodiments, water-in-oil emulsions can be usedas a fluid-loss control agent as the droplets can be made big enough toplug the formation pores. This emulsion can be used in fracturing fluidor the pad to give the fluid-loss characteristic. The breaker inside theemulsion can break the emulsion and thus release the obstruction ofpores after the job.

Acidizing Fluid and Acidizing

The nanohybrid can be used in the acidizing of carbonate rock. Thenanohybrid can be used to make 15% HCl emulsion in oil (continuousphase) and can be pumped in the carbonate formation as a kind of matrixtreatment. This nanohybrid-stabilized emulsion will provide acid systemsthat will slowly etch the carbonate formation and will help in making alonger fracture. Optionally polymers that gel the aqueous phase of theemulsion can be used to prevent the leakage of acid prematurely.

Controlling acid reaction rates by incorporation of the acid as aninternal phase in an emulsion is well known. For ultra hot wells, suchas the Khaff in Saudi Arabia, the conventional acid gel systems are notas temperature stable as the nanohybrid-stabilized emulsions would be,and thus new high-temperature acid retardation can be obtained.

The emulsion can be broken when desired by modifying the nanohybrid. Forexample, the aqueous phase can be loaded with an oxidizer to self-breakthe emulsion after a delay. Other examples of techniques for breakingthe emulsion stabilized with a nanohybrid are described above.

Fracturing Fluid and Fracturing

Nanohybrid emulsions can be used to produce viscous fracturing fluidsthrough emulsification that are stable at temperatures not currentlyobtainable even with synthetic polymers. The emulsion can be broken asdesired, breaking at least some of the viscosity of the fluid. Besidesapplications at high temperature (up to 250° C.), the resultingconductivity impairment from the broken emulsion would be negligible,providing a benefit relative to most polymer thickened materials.

In addition, if the water phase of the emulsion is gelled, as in theHalliburton SuperEmulsifrac™ system, and the encapsulation feature ofthe invention is used (described below), it would provide a two-phaseviscous system and a delayed crosslink feature, too.

For a fracturing application, a high internal phase emulsion (“HIPEemulsion”), which is water-in-oil, can be made with the nanohybridsystem. High internal phase water-in-oil emulsions are defined as havinggreater than about 80% dispersed aqueous phase. These emulsions arecapable of carrying proppants.

Fluid for Frac Packing or Gravel Packing

The nanohybrid can be used to stabilize an emulsion for use in “fracpacking” or “gavel packing” operations. The emulsion can be broken asdesired, breaking at least some of the viscosity of the fluid. Again,the nanohybrid-stabilized emulsion has applications at high temperature(up to 250° C.), the resulting conductivity impairment from the brokenemulsion would be negligible, providing a benefit relative to mostpolymer thickened materials.

Emulsion Encapsulation for Delayed Chemical Release for Use in a WellApplication

A nanohybrid can also be used to encapsulate a chemical in an internalphase of an emulsion and then selectively break the emulsion to releasethe chemical in a controlled manner for use in a well operation, such asdrilling, cementing, or treatment.

For example, a water-soluble chemical can be encapsulated in theinternal water phase of a water-in-oil emulsion stabilized with ananohybrid. The water-in-oil emulsion can then be emulsified in an outerwater phase or aqueous fluid, either with a suitable conventionalemulsifier or with a suitable nanohybrid. Upon breaking of thewater-in-oil emulsion, the water-soluble chemical is released from theoil of the water-in-oil emulsion into the outer water phase.

Similarly, an oil-soluble chemical can be encapsulated in the internaloil phase of an oil-in-water emulsion stabilized with a nanohybrid. Theoil-in-water emulsion can then be emulsified in an outer oil phase,either with a suitable conventional emulsifier or with a suitablenanohybrid. Upon breaking the oil-in-water emulsion, the oil-solublechemical is released from the water of the oil-in-water emulsion intothe outer oil phase. For example, esters can be solubilized in oil phaseof the emulsion and then they hydrolyze to provide organic acids thatcan be used to break crosslinked fluid, solubilize calcium carbonate infilter cake to break filter cake and similar applications.

In an embodiment, the nanohybrid of the present invention can be dopedwith a crosslinker for a fracturing fluid. The rate of release would beadjusted to obtain the proper crosslink time. For example, as afracturing fluid heats in a formation, being able to deliver viacontrolled release additional crosslinker or a different crosslinkerwould provide benefits in a fracturing treatment.

In another embodiment, a water-soluble breaker (for example, an enzyme,oxidizer, acid, etc.) for a crosslinked gel can be contained in theinternal water phase of a water-in-oil emulsion stabilized with ananohybrid. The water-in-oil emulsion is itself emulsified in an outeraqueous phase. Upon breaking the water-in-oil emulsion, the breaker isreleased from inside the oil phase to break an aqueous fracturing gel ofthe outer-aqueous phase after a desired delay.

Similarly, these techniques can be employed to encapsulate a breaker fora filter cake to help break the filter cake from the inside.Polymer-based fluid-loss control pills often require long cleanupperiods. Moreover, an effective cleanup usually requires fluidcirculation to provide high driving force, which allows diffusion totake place to help dissolve the concentrated build up of materials. Suchfluid circulation may not be feasible. Additional methods of deliveringor releasing a chemical to help remove a filter cake are desirable.

Viscous Sweep Application

Emulsions made by the nanohybrid can also be used to form a viscousfluid for viscous sweeps. In this application, a relatively small volumeof viscous fluid is circulated to sweep, or remove, debris or residualfluids from the circulation system. The viscosity of the fluid can bebroken by modifying the nanohybrid.

Spacer Fluid Application

A nanohybrid-stabilized emulsion can be used in a spacer fluid, forexample, in a cement spacer. The nanohybrid can be used to control theviscosity of the spacer without the need for any polymer, can be brokenon demand, and also the desired aqueous phase can be weighted with clearfluids, or the internal phase of the emulsion set to yield the desiredviscosity. The high temperature stability of the emulsion allowsachievement of higher temperatures than currently capable conventionalpolymers.

Swellable Packer Application

Rubber swellable packers are used to close the annulus between formationand the pipe. An oil-in-water emulsion stabilized according to theinvention can be used to swell a swellable packer having an oil-swellingrubber element after a delay. A water-in-oil emulsion can be used toswell a swellable packer having a water-swelling packing material aftera delay. A delay is desired for positioning the packer at theappropriate location in a wellbore before swelling the packer. Afterplacing the swell packer the emulsion is broken by the methods describedso that the internal phase can swell the packer and set it in place.

CONCLUSIONS

The nanohybrid emulsions are very stable, including, for example, athigh temperatures (up to 250° C.), over a broad pH range, withhigh-ionic strength aqueous phases, and to high dilution. In addition,the nanohybrid can be tailored to have desired HLB for making emulsionsuseful in various applications. According to the methods, the emulsionis chemically degraded or completely broken downhole.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is, therefore, evident thatthe particular illustrative embodiments disclosed above may be alteredor modified and all such variations are considered within the scope andspirit of the present invention. While compositions and methods aredescribed in terms of “comprising,” “containing,” or “including” variouscomponents or steps, the compositions and methods also can “consistessentially of” or “consist of” the various components and steps.Whenever a numerical range with a lower limit and an upper limit isdisclosed, any number and any included range falling within the range isspecifically disclosed. In particular, every range of values (of theform, “from about a to about b,” or, equivalently, “from approximately ato b,” or, equivalently, “from approximately a to b”) disclosed hereinis to be understood to set forth every number and range encompassedwithin the broader range of values. Also, the terms in the claims havetheir plain, ordinary meaning unless otherwise explicitly and clearlydefined by the patentee. Moreover, the indefinite articles “a” or “an”,as used in the claims, are defined herein to mean one or more than oneof the element that it introduces. If there is any conflict in theusages of a word or term in this specification and one or more patent(s)or other documents that may be incorporated herein by reference, thedefinitions that are consistent with this specification should beadopted.

What is claimed is:
 1. A method of using a nanohybrid-stabilizedemulsion in a well, wherein the method comprises the steps of: (a)forming an emulsion comprising: (i) a nanohybrid comprising a carbonnanotube attached to a hydrophilic inorganic second component; (ii)water or an aqueous solution; and (iii) a water-immiscible liquid; (b)introducing a well fluid comprising the emulsion into the well; and (c)after the step of introducing, modifying the nanohybrid to break theemulsion in the well.
 2. The method according to claim 1, wherein thenanohybrid has a hydrophilic-lipophilic balance adapted to forming anoil-in-water emulsion.
 3. The method according to claim 1, wherein thenanohybrid has a hydrophilic-lipophilic balance adapted to forming awater-in-oil emulsion.
 4. The method according to claim 1, wherein theemulsion further comprises a polymer or other particles to helpstabilize the emulsion.
 5. The method according to claim 1, wherein thewater-immiscible liquid is water insoluble.
 6. The method according toclaim 1, wherein the emulsion is stable at 77° F. for 10 days.
 7. Themethod according to claim 1, wherein the emulsion is stable tohigh-dilution conditions when tested at 77° F.
 8. The method accordingto claim 1, wherein the well fluid introduced into the well is theemulsion.
 9. The method according to claim 1, wherein the well fluid isintroduced into the well at a rate and pressure and directed to asubterranean formation at a rate and pressure that is at leastsufficient to create at least one fracture in the subterranean formationof the well.
 10. The method according to claim 1, wherein the well fluidfurther comprises a proppant.
 11. The method according to claim 1,wherein the well fluid is selected from the group consisting of adrilling fluid, a cementing composition, a fluid-loss control pill, anacidizing fluid, a viscous-sweep fluid, a fracturing fluid, afrac-packing fluid, a gravel-packing fluid, a spacer fluid, and a fluidfor swelling a swellable packer.
 12. The method according to claim 1,wherein the well fluid comprises a chemical for modifying thehydrophilic-lipophilic balance of the nanohybrid.
 13. The methodaccording to claim 1, wherein the step of modifying the nanohybridcomprises the use of an oxidizer.
 14. The method according to claim 1,wherein the step of modifying the nanohybrid comprises the use of acompound that forms a hydrophilic group to be attached to thenanohybrid, which causes the emulsion to break.
 15. The method accordingto claim 1, wherein the step of modifying the nanohybrid compriseswrapping the nanohybrid with a polymer.
 16. The method according toclaim 1, wherein the hydrophilic inorganic second component of thenanohybrid is silica.
 17. The method according to claim 1, wherein thehydrophilic inorganic second component of the nanohybrid is selectedfrom the group consisting of: alumina, magnesium oxide, and titaniumoxide.
 18. The method according to claim 1, wherein a dispersed phase ofthe emulsion comprises a chemical to be released from the dispersedphase upon the step of modifying the nanohybrid to break the emulsion.19. The method according to claim 18, wherein the chemical to bereleased comprises at least one additive selected from the groupconsisting of a retarder or accelerator for cement, a corrosioninhibitor, an acid for acidizing, a crosslinker for a polysaccharide,and a breaker for polysaccharide.
 20. The method according to claim 1,wherein the hydrophilic inorganic second component of the nanohybridcomprises silica and the step of modifying the nanohybrid comprises theuse of a long chain quaternary amine to modify the silica of thenanohybrid.
 21. The method according to claim 20, wherein the step ofmodifying the nanohybrid comprises functionalizing the silica withsilane to make it hydrophobic.
 22. The method according to claim 1,further comprising the step of mixing the emulsion with a third fluid toform the well fluid prior to or during the step of introducing the wellfluid into the well.
 23. The method according to claim 22, wherein thewell fluid introduced into the well comprises the emulsion as a nestedemulsion in the third fluid, wherein the third fluid is the continuousphase of the well fluid as a whole.
 24. The method according to claim23, wherein the third fluid comprises water and a water-solublepolysaccharide.
 25. The method according to claim 24, wherein the thirdfluid comprises at least a sufficient concentration of the water-solublepolysaccharide to be capable of forming a crosslinked gel upon mixingwith a crosslinker.
 26. The method according to claim 22, wherein thethird fluid comprises water and one or more inorganic ionic salts.